Add ABOUT file and provide credits in the README, also flesh out README

[pombredanne]

I think we want some ABOUT file https://github.com/aboutcode-org/cyseq/blob/main/src/cyseq/_seq.pyx to trace the code origin. Also giving some credit to Python and Cydifflib on the main README is a must.

[pombredanne]

License-wise, the sitution need some clarification

1. ScanCode original licensedcode/seq.py is under the Python-2.0 license since it is derived from upstream difflib.py as explained in

• https://github.com/aboutcode-org/scancode-toolkit/blob/develop/src/licensedcode/seq.py.ABOUT
• https://github.com/aboutcode-org/scancode-toolkit/blob/develop/src/licensedcode/seq.py#L9

2. Cydifflib is under the MIT license but since this is all derived from the upstream Python-2.0-licensed difflib.py, it is not clear if the MIT-licensing is entirely correct.

• MIT license https://github.com/rapidfuzz/CyDifflib/blob/main/LICENSE
• Diff between upstream and Cydifflib port shows that the code is clearly derived and mostly similar to upstream
  • https://gist.github.com/pombredanne/5f90bb1b3f9afa2f3daa70b5dbe7b7eb/revisions?diff=split&w

3. The rest of the code in cyseq is overall Apache-licensed.